The Great Oxygenation Event, the beginning of which is commonly known in scientific media as the Great Oxidation Event (GOE, also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, Oxygen Revolution, or Great Oxidation) was the biologically induced appearance of dioxygen (O2) in Earth's atmosphere. Geological, isotopic, and chemical evidence suggest that this major environmental change happened around 2.45 billion years ago (2.45 Ga), during the Siderian period, at the beginning of the Proterozoic eon. The causes of the event are not clear. The current geochemical and biomarker evidence for the development of oxygenic photosynthesis before the Great Oxidation Event has been mostly inconclusive.
Oceanic cyanobacteria, which evolved into coordinated (but not multicellular or even colonial) macroscopic forms more than 2.3 billion years ago (approximately 200 million years before the GOE), are believed to have become the first microbes to produce oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE started when these oxygen sinks became saturated, at which point oxygen produced by the cyanobacteria was free to escape into the atmosphere.
The increased production of oxygen set Earth's original atmosphere off balance. Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have destroyed most such organisms at the time. Cyanobacteria were therefore responsible for one of the most significant mass extinctions in Earth's history. Besides marine cyanobacteria, there is also evidence of cyanobacteria on land.
A spike in chromium contained in ancient rock deposits formed underwater shows the accumulation had been washed off from the continental shelves. Chromium is not easily dissolved and its release from rocks would have required the presence of a powerful acid. One such acid, sulfuric acid (H2SO4), might have been created through bacterial reactions with pyrite. Mats of oxygen-producing cyanobacteria can produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice, and before oxygen started accumulating in the atmosphere, these organisms would already be adapted to oxygen. Additionally, the free oxygen would have reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest episode of glaciation in Earth's history and called snowball Earth.
The most widely accepted chronology of the Great Oxygenation Event suggests that free oxygen was first produced by prokaryotic and then later eukaryotic organisms that carried out oxygenic photosynthesis more efficiently. The excess of oxygen those organisms produce is a waste product. These organisms lived long before the GOE, perhaps as early as .
Initially, the oxygen they produced would have quickly been removed from the atmosphere by the chemical weathering of reducing minerals (those able to be oxidized), most notably iron. This 'mass rusting' led to the deposition of iron(III) oxide in the form of banded-iron formations such as the sediments in Minnesota and Pilbara, Western Australia. Oxygen only began to persist in the atmosphere in small quantities shortly (~50 million years) before the start of the GOE. Without a mineral sink in the form of iron, oxygen could have accumulated very rapidly. For example, at today's rates of photosynthesis (which are much greater than those in Precambrian when there were no land plants), modern atmospheric O2 levels could be produced in around 2,000 years.
Another hypothesis is that oxygen producers did not evolve until a few million years before the major rise in atmospheric oxygen concentration. This is based on a particular interpretation of a supposed oxygen indicator, the mass-independent fractionation of sulfur isotopes, a marker used in previous studies. This hypothesis would eliminate the need to explain a lag in time between the evolution of oxyphotosynthetic microbes and the rise in free oxygen.
In either case, oxygen did eventually accumulate in the atmosphere, with two major consequences.
Firstly, it oxidized atmospheric methane (a strong greenhouse gas) to carbon dioxide (a weaker one) and water. This decreased the greenhouse effect of the Earth's atmosphere, causing planetary cooling, and triggered the Huronian glaciation. Starting around 2.4 billion years ago, this lasted 300-400 million years, and may have been the longest ever snowball Earth episode.
Secondly, the increased oxygen concentrations provided a new opportunity for biological diversification, as well as tremendous changes in the nature of chemical interactions between rocks, sand, clay, and other geological substrates and the Earth's air, oceans, and other surface waters. Despite the natural recycling of organic matter, life had remained energetically-limited until the widespread availability of oxygen. This breakthrough in metabolic evolution greatly increased the free energy availability for living organisms, with truly global environmental impacts. For example, mitochondria evolved after the GOE, and with more energy available from oxygen, organisms had the means for new, more complex morphologies. These new morphologies in turn helped drive evolution through increasingly complex interactions between organisms.
The gap between the start of oxygen production from photosynthetic organisms and the geologically rapid increase in atmospheric oxygen (about 2.5–2.4 billion years ago) may have been as long as 900 million years. Several hypotheses might explain the time lag:
The oxygen increase had to await tectonically driven changes in the Earth, including the appearance of shelf seas, where reduced organic carbon could reach the sediments and be buried. The newly produced oxygen was first consumed in various chemical reactions in the oceans, primarily with iron. Evidence is found in older rocks that contain massive banded iron formations that were apparently laid down as this iron and oxygen first combined; most of the planet's commercial iron ore is in these deposits. Researchers found that the amount of oxygen in the air spiked each time smaller land masses collided to form a super-continent. These massive pile ups generated mountain chains and as these mountains eroded, they released nutrients into the ocean, feeding the cyanobacteria that carry out photosynthesis.
The early chemosynthetic organisms would have been a source of methane, which is an important trap for molecular oxygen, because oxygen readily oxidizes methane to carbon dioxide (CO2) and water in the presence of UV radiation. Modern methanogens require nickel as an enzyme cofactor. As the Earth's crust cooled, the supply of nickel from volcanoes was reduced and hence less methane was produced. This allowed the oxygen percentage of the atmosphere to rise as the decrease in production of methane allowed the oxygen producing algae and other such life forms to, in a sense, out-produce the methane producers. From 2.7 to 2.4 billion years ago, the rate of deposition of nickel declined steadily; it was originally 400 times today's levels.
Another hypothesis posits a model of the atmosphere that exhibits bistability in oxygen concentrations. In this model, UV shielding decreases the rate of methane oxidation once oxygen levels are sufficient to support the formation of an ozone layer. This explanation proposes an atmospheric system experiencing two steady states, one with lower (0.02%) atmospheric oxygen content, and the other with higher (21% or more) oxygen content. The Great Oxygenation Event can then be understood as a switch between the lower to the upper stable steady states.
Another factor is the presence of hydrogen gas. The appearance of cyanobacteria might explain the decline of hydrogen gas and why Earth's air is so oxygen-rich.
Some bacteria living in the early oceans had the ability to separate water into hydrogen and oxygen molecules. By using the energy of the Sun, the separate hydrogen molecules were able to create organic compounds, and oxygen was a by-product. If the hydrogen-heavy compounds were buried, it would have allowed oxygen to accumulate in the atmosphere.
However, in 2001 scientists realized that the hydrogen would instead escape into space through a process called methane photolysis, in which hydrogen-containing methane reacts with oxygen releasing hydrogen. This hypothesis could explain why the early Earth stayed warm enough to sustain oxygen producing lifeforms.
There is a possibility that the oxygen indicator was misinterpreted. During the proposed time of the lag in the previous theory, there was a change from mass-independently fractionated (MIF) sulfur to mass-dependently fractionated (MDF) sulfur in sediments. This was assumed to be a result of the appearance of oxygen in the atmosphere (since oxygen would have prevented the photolysis of sulfur dioxide, which causes MIF). This change from MIF to MDF of sulfur isotopes also may have been caused by an increase in glacial weathering, or the homogenization of the marine sulfur pool as a result of an increased thermal gradient during the Huronian glaciation period.
The Great Oxygenation Event triggered an explosive growth in the diversity of minerals on Earth. This now meant that many elements could occur in one or more oxidized forms in the near-surface environment. It is estimated that the Great Oxygenation Event alone was directly responsible for more than 2,500 new minerals of the total of about 4,500 minerals found on Earth. Most of these new minerals were formed after the Great Oxygenation event as hydrated and oxidized forms due to dynamic mantle and crust processes.